In recent years, printed electronics using printing technology have attracted attention in the field of electronic devices. In particular, conductive inks are recognized as one of industrially very important technologies because conductive lines can be formed by applying conductive inks by coating processes and such coating processes can provide a more significant reduction in cost and environmental impact than conventional vacuum processes. Thus, materials for such conductive inks are under active development.
An example of conventional conductive inks is a metal paste obtained by mixing metal particles having a size on the order of micrometers with a binder resin and a solvent. Such metal pastes are widely used in electronic products such as printed circuit boards. The metal pastes, however, must be sintered at 200° C. to 300° C. to exhibit electrical conductivity. Therefore, how to achieve high electrical conductivity by sintering at lower temperatures is a challenge to be addressed. In addition, due to their high viscosities, conventional metal pastes cannot be used in certain printing techniques such as inkjet printing.
In order to solve these problems, there have been recently developed metal nanoparticle inks capable of exhibiting high electrical conductivities of 10−5 Ω·cm or less when sintered at temperatures of 150° C. or lower. Metal nanoparticles contained in such an ink have a structure in which a metal in the form of nanoparticles is coated with protective organic molecules acting as a surfactant. The action of the protective organic molecules allows the metal nanoparticles to be dispersed relatively stably in various organic solvents. Some of the protective organic molecules are desorbed from the metal nanoparticles even at room temperature. Therefore, the resulting metal nanoparticles are readily sintered together and thus exhibit high electrical conductivity even when sintered at low temperatures. With the use of metal nanoparticles having such characteristics, it is possible to use a low-temperature process to form an electronic circuit even on a plastic film having a low heat-resistance temperature.
On the other hand, when such a conductive ink is used to form conductive lines of an electronic circuit, the adhesion between the conductive lines and a substrate is an important factor to ensure the reliability of the resulting product. Since conventional metal pastes contain a binder resin having a molecular structure that enhances the adhesion between conductive lines and a substrate, they can provide strong adhesion between them. In addition, since such conventional metal pastes are resistant to high-temperature sintering, they have an advantage of being able to provide better adhesion between the conductive lines and the substrate. In contrast, metal nanoparticles contain fewer protective organic molecules. Therefore, such metal nanoparticles exhibit high electrical conductivity even at low temperatures, but the use of such metal nanoparticles makes it difficult to enhance the adhesion between conductive lines and a substrate. So in order to form highly reliable conductive lines, improvement of adhesion between a substrate and conductive lines formed using metal nanoparticles is a challenge to be addressed.
There have been many studies and some proposals to enhance the adhesion between substrates and metal thin films formed using metal nanoparticle inks. For example, Patent Literature 1 describes a method for sintering a metal nanoparticle ink by irradiating the ink with a laser beam. Patent Literature 2 describes the formation of a thin film from a metal fine powder paste on a glass substrate which has been surface-treated with a silane coupling agent. Patent Literature 3 describes the use of metal nanoparticles containing a linear epoxy resin.